Which matrix can be used to solve the system? x+y=21 5x+4y=20 a) [x]=[4 -1][21] [y]=[-5 1][20] b) [x]=[-4 1][21] [y]=[5 -1][20] c) [x]=[-1 1][21] [y]=[5 -4][20] d) [x]=[1 1][21] [y]=[5 4][20] if there is any confusion on what anything written here is here is a screenshot of everything.

It's the last choice.  In the top row you have the 1's that are the coefficients on the x and the y, and the solution for that equation is 21; in the second row you have the 5 from the x and the 4 from the y and the solution for that equation which is 20.  I'm more used to writing them down instead of across, but there you have it anyways